One type of device used for protecting the rotor winding is known, for example, from the article "Electronic Rotor Earth Fault Protection with Novel Measuring Principle" by E. F. Knutter and G. Ziegler in Siemens Journal 46 (1972), Volume 12, pages 906 to 909.
In principle, a single ground fault somewhere in the rotor circuit which, apart from the rotor winding, also includes the exciter source, among others, does not immediately affect the operational characteristics of the machine. It is, therefore, not mandatory in every case to switch the machine off. If the ground fault has occurred, for example, in the exciter source, the machine can be possibly kept in operation until the next routine shutdown. On the other hand, in the case of a ground fault right in the rotor winding, it is desirable to switch the machine off since a further ground fault in the rotor winding could short-circuit a part of the winding through ground and lead to a destruction of the rotor winding. The prerequisite for such a selective shutdown is that the ground fault is detected selectively in accordance with its position in the rotor circuit. The known device basically does not indicate ground faults selectively in the total rotor circuit.